In communications apparatuses such as cell phones, smart phones, and wireless LANs, and electronic apparatuses such as computers, signals in as wide frequency ranges as from several MHz to several GHz are treated, generating electromagnetic wave noises in wide frequency ranges. Electromagnetic wave noises generated from communications apparatuses and electronic apparatuses should be reduced, circuits in the communications apparatuses and electronic apparatuses should be protected from external electromagnetic wave noises, and electromagnetic wave noises generated from individual circuit devices should be prevented from adversely affecting other circuit devices.
Electromagnetic shielding technologies are generally used for to electromagnetic wave noises. By the electromagnetic shielding technologies, noise sources and noise-receiving parts are surrounded by metal plates to shield electromagnetic wave noises. For example, with metal shield plates disposed in the casings of communications apparatuses and electronic apparatuses, electromagnetic wave noises generated therefrom are suppressed. However, because electromagnetic wave noises inside the apparatuses are not reduced, the metal shield plates do not sufficiently reduce noises for the mounted parts. Thus, what is desired are not electromagnetic wave shields reflecting electromagnetic wave noises, but electromagnetic wave-absorbing sheets capable of absorbing electromagnetic wave noises.
JP 2010-153542 A discloses a electromagnetic wave noise suppression sheet comprising a substrate, a conductive layer formed by coating a conductive paint, and a magnetic layer formed by coating a magnetic paint. Specific examples of the substrate include papers, non-woven or woven fabrics, cloths, resin sheets, etc. The conductive paint contains a metal such as copper, gold, aluminum, etc. or a conductive material such as carbon, etc. The magnetic paint contains particles of metal oxides such as soft-magnetic ferrite and a magnetic metal such as Sendust, Permalloy, and amorphous alloys. This electromagnetic wave noise suppression sheet has electromagnetic wave absorbability improved by both of the conductive layer and the magnetic layer. However, such combination of the conductive layer and the magnetic layer fails to provide sufficient electromagnetic wave absorbability.